Layers of the aforementioned type are already known from the prior art. Layers of this type are used as separators in batteries and capacitors, which have the function of energy storage. The storage of a charge in batteries and capacitors takes place chemically, physically or in a mixed form, for example by chemisorption.
To avoid an internal discharge within the battery or capacitor, electrodes of opposing charge are separated from one another mechanically by materials which do not conduct electrons known as separators or spacers. At the same time, the separators or spacers allow the transport of ionic charge carriers of an electrolyte between the electrodes owing to their porosity which is adapted to the energy storage system and the application thereof.
The separators known from the prior art have small, mutually crosslinked openings of about a micrometer. These openings should be as large as possible so that the electrolyte conductivity in the impregnated separator is as high as possible and the battery therefore has a high power density. If the openings are too large however, metal dendrites can lead to a short circuit between the two electrodes which are actually to be electrically separated from one another. The metal dendrites consist either of lithium or of other metals which can be in the form of impurities in the battery.
In addition, particles of electrically conductive electrode materials can migrate through the openings. Owing to these processes, a short circuit can occur between the electrodes and the spontaneous discharge of the battery or the capacitor can be markedly accelerated.
Locally very high currents can flow during a short circuit, leading to the liberation of heat. This heat can cause the separator to melt with the result that the insulating effect of the separator can in turn decrease significantly. A battery which runs down very rapidly represents a high safety risk owing to its high energy content and the combustibility of the electrolyte and other constituents.
A further drawback of the separators known from the prior art is their inability to withstand rising temperatures. The melting point is about 130° C. when polyethylene is used and 150° C. when polypropylene is used.
Possible causes of short circuits include shrinkage of the separator due to an excessively high temperature in the battery, growth of metal dendrites due to a reduction of metal ions (lithium, iron, manganese or other metal impurities), abrasion of electrode particles, cutting abrasion or broken electrode coating and direct contact of the two flat electrodes under pressure.
EP 0 892 448 A2 discloses what is known as the shut-down mechanism, which counteracts the planar propagation of local heating, for example due to a short circuit, in that the ion conduction is prevented in the vicinity of the initial short circuit. Owing to the heat evolved by the short circuit, polyethylene is heated to such an extent that it melts and closes the pores of the separator. High melting point polypropylene remains mechanically intact.
US 2002/0168569 A1 describes the construction of a separator consisting of polyvinyl fluoride, which is partially dissolved by a solvent in the production process, is blended with silicon dioxide particles and is applied as a thin film. A porous membrane remains when the solvent is removed.
WO 2006/068428 A1 describes the production of separators for lithium ion batteries using a polyolefin separator which is additionally filled with gel-like polymers and inorganic particles.
WO 2004/021475 A1 describes the use of ceramic particles which are shaped into a thin planar product from oxides of the elements silicon, aluminium and/or zirconium, via organo-silicon adhesives and inorganic binders.
In order to establish sufficient mechanical flexibility, the ceramic particles are introduced into a supporting material, for example a non-woven fabric. This is disclosed in WO 2005/038959 A1.
The use of low-melting-point waxes as an admixture to a ceramic paste in order to prevent short circuits in the early stage of metal dendrite formation is described in WO 2005/104269 A1.
WO 2007/028662 A1 describes the addition of polymer particles with a melting point of over 100° C. to ceramic fillers in order to improve the mechanical properties of the separator. The described materials are to function as a separator for lithium ion materials. Although these separators lead to higher resistance to heat than membranes, they cannot be used commercially. This is due, on the one hand, to the relatively high costs and, on the other hand, to the excessive thickness of the material, which is greater than 25 μm.
WO 2000/024075 A1 describes the production of a membrane which can be used in fuel cells. It consists of glass fibre materials in which fluorocarbon polymers are fixed by means of a silicate binder.
Finally, JP 2005268096 A describes a separator for Li-ion batteries which is produced by melting together thermoplastic particles in a polyethylene/polypropylene fibrous supporting material by heating. This separator has a bubble-like pore structure with a pore diameter of 0.1-15 μm.
European patent EP 1 138 092 B1 relates to composite bodies for electrochemical cells. These composite bodies consist of two layers, a layer of particles and binder being applied to a second layer. The second layer can be a film or non-woven fabric.
US 2006/0078722 A1 discloses porous films of a crosslinked polyolefin resin.
EP 1 271 673 A1 relates to gas-permeable separators for batteries. In this case, a crosslinked polymer layer is applied to a porous substrate.
However, the prior art does not reveal a cost-efficient separator which has high porosity and high heat resistance while having a low thickness and which can be used over a broad temperature range with high safety requirements in batteries with a high power and energy density. A particular problem of known materials is that they shrink at elevated temperature, thereby reducing porosity and adversely affecting the properties. In addition, there are no methods for producing efficient separators simply in a few operating steps.